Its time to replace my 2 T5 tubes in my main tank. Juwel seem to make three types for freshwater; Day, Nature, and Colour. My main requirement is light that is good for plant growth. What effect would the different combos have?
All 3 of the above lights are different but have the same colour spectrum. However, the 8000K has significantly more blue light to encourage coral growth. The 4100K has the least amount of blue light but still has some. The 6400K has more blue light than the 4100, but less blue light than the 8000. The 6400K globe would be my choice.
You might try a combo of 6400K and 4100K. However, make sure they will take the 4100K back in exchange for a second 6400K if it is not adequate. If they won't exchange, I would not risk it, get two 6400K.
I use T8 fluorescent, and on my dual tube tanks I use one 6500K and one 5500K for a tad more warmth (red). But I have tried this sort of combo with a 6500K and a 4000K and it was not as good.
On another issue, T5 is very bright light for freshwater fish, so a good cover of floating plants is advisable (or, like many high-light planted tanks, just plants and no fish).